The invention relates to a gas and steam-turbine plant having a heat-recovery steam generator, which is connected downstream of a gas turbine on the flue-gas side and which has heating surfaces that are connected in a water/steam circuit of a steam turbine. A gasifier for fuel is connected upstream of a combustion chamber of the gas turbine.
A gas and steam-turbine plant having integrated gasification of fossil fuel normally includes a gasifier for the fuel. That gasifier is connected on the outlet side to the combustion chamber of the gas turbine through a number of components provided for gas cleaning. In that case, a heat-recovery steam generator has heating surfaces connected in the water/steam circuit of the steam turbine and may be connected downstream of the gas turbine on the flue-gas side. Such a plant has been disclosed, for example, by UK Patent Application GB 2 234 984 A.
Furthermore, German Published, Non-Prosecuted Patent Application DE 33 31 152 A1 discloses a method of operating a gas-turbine plant having a fuel-gasification plant. In that method, low-oxygen air collecting in an air-separation unit is admixed to fuel of medium calorific value delivered to the fuel-gasification plant and the fuel/air mixture of low calorific value is fed to a combustion chamber of the gas-turbine plant. In that case, the compressor of the gas-turbine plant, in addition to supplying the combustion chamber with air, also supplies the air-separation unit with air. It is known from U.S. Pat. No. 4,677,829 and U.S. Pat. No. 4,697,415 to cool compressed air from an air compressor through the use of heat exchangers.
A device for removing sulphurous constituents is provided in that plant in order to provide reliable cleaning of the gasified fossil fuel. A saturator is connected downstream of that device in a feed line, opening into the combustion chamber, for the gasified fuel. The gasified fuel is loaded with steam in the saturator in order to reduce pollutant emission. To that end, the gasified fuel flows through the saturator in counterflow to a water flow, which is directed in a water circuit referred to as a saturator circuit. In order to provide an especially high efficiency, provision is made for an input of heat from the water/steam circuit into the saturator circuit.
In addition to the fossil fuel, oxygen required for the gasification of the fuel can also be fed to the gasifier of such a gas and steam-turbine plant. In order to obtain that oxygen from air, an air-separation unit is normally connected upstream of the gasifier. In that case, a partial flow, also referred to as bleed air, of air compressed in an air compressor associated with the gas turbine, may be admitted to the air-separation unit.
As a result of the compression process, the air flowing off from the compressor has a comparatively high temperature level. Cooling of the partial flow of the compressed air, also referred to as bleed air, is therefore normally necessary before it enters the air-separation unit. The heat extracted from the bleed air in the process is normally transferred to the saturator circuit for heat recovery and thus for achieving a high plant efficiency. Depending on the operating state of the plant, only residual cooling of the bleed air through the use of cooling water before it enters the air-separation unit is then necessary in such a structure.
However, such a concept for cooling the bleed air assumes that the heat supply during the air cooling and the heat demand in the saturator circuit are matched to one another in a sufficiently effective manner. Depending on the integration concept, that is depending on the type of air supply for the air-separation unit and the components used in the process, such bleed-air cooling therefore cannot be used universally and is only reliable to a limited extent in some operating states of the gas and steam-turbine plant.
It is accordingly an object of the invention to provide a gas and steam-turbine plant, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which, irrespective of an integration concept taken as a basis, reliable cooling of bleed air, in an especially simple type of construction, is ensured in all operating states.
With the foregoing and other objects in view there is provided, in accordance with the invention, a gas and steam-turbine plant, comprising a steam turbine having a water/steam circuit, a gas turbine having a flue-gas side, a combustion chamber associated with the gas turbine, an air compressor associated with the gas turbine for supplying a partial flow of air compressed in the air compressor, and a bleed-air line connected to the air compressor. An air-separation unit supplying oxygen and having an inlet side connected to the bleed-air line receives the partial flow of air compressed in the air compressor. A heat-recovery steam generator is connected downstream of the gas turbine on the flue-gas side. The steam generator has heating surfaces connected in the water/steam circuit. A gasifier for fuel is connected upstream of the combustion chamber and receives oxygen from the air-separation unit. A heat exchanger is constructed as an evaporative cooler for cooling the partial flow of compressed air. The heat exchanger has a primary side connected to the bleed-air line and a secondary side connected to a water/steam drum to form an evaporator circuit for a flow medium.
The invention is thus based on the concept that, for bleed-air cooling which can be used irrespective of the integration concept and the fuel to be gasified and is reliable in all operating states, the heat extracted from the bleed air should be capable of being drawn off irrespective of a firmly preset heat demand. The bleed-air cooling should therefore be isolated from the heat supply into the saturator circuit. The bleed air is instead cooled by heat exchange with a flow medium. In this case, evaporation of the flow medium is provided for especially high operational stability in a simple type of construction and for favorable input of the heat extracted from the bleed air into the plant process.
In accordance with another feature of the invention, in order to provide especially flexible bleed-air cooling which can be adapted to various operating states in a simple manner, a further heat exchanger, constructed on the secondary side as an evaporator for a flow medium, is connected downstream of the heat exchanger in the air-bleed line. The heat exchanger is constructed as an intermediate-pressure evaporator and the further heat exchanger is constructed as a low-pressure evaporator.
The heat exchanger constructed as an intermediate-pressure evaporator is expediently connected on the flow-medium side to a heating surface, associated with an intermediate-pressure stage of the steam turbine, in the heat-recovery steam generator. In an analogous configuration, the heat exchanger constructed as a low-pressure evaporator may be connected on the flow-medium side to a heating surface, associated with a low-pressure stage of the steam turbine, in the heat-recovery steam generator. However, the heat exchanger constructed as a low-pressure evaporator is expediently connected on the flow-medium side to a secondary steam consumer, for example to the gasifier or to a gas-processing system connected downstream of the latter. In such a configuration, reliable feeding of the secondary consumer with process steam or with heating steam is ensured in an especially simple manner.
In accordance with a further feature of the invention, at least one of the heat exchangers is connected on the secondary side to a water/steam drum in order to form an evaporator circuit.
In this case, the evaporator circuit may be constructed with forced circulation. However, in an especially advantageous development, the respective evaporator circulation is constructed with natural circulation. Circulation of the flow medium is ensured by pressure differences occurring during the evaporation process and/or by a geodetic configuration of the evaporator and the water/steam drum. In such a configuration, only a circulating pump with a comparatively low rating is required for starting the evaporator circulation. In accordance with an added feature of the invention, the respective water/steam drum is expediently connected to a number of heating surfaces disposed in the heat-recovery steam generator.
In accordance with an additional feature of the invention, there is provided an additional heat exchanger connected downstream of the heat exchanger in the bleed-air line. The additional heat exchanger is connected on the secondary side to a feedwater tank associated with the heat-recovery steam generator. With such a configuration, an especially favorable input of heat into the saturator circuit can be achieved, with the input of heat being independent of the integration concept. This is because, in this case, the input of heat into the saturator circuit can be effected through a heat exchanger, through which preheated feedwater extracted from the feedwater tank can flow on the primary side. The feedwater leaving this heat exchanger and cooled down by the input of heat into the saturator circuit can then be fed to the additional heat exchanger connected in the bleed-air line, where it heats up again due to the further cooling of the bleed air. An input of heat into the saturator circuit can therefore be achieved without greater heat losses in the feedwater.
In accordance with a concomitant feature of the invention, there is provided a cooling-air line which branches off from the bleed-air line downstream of the heat exchanger or downstream of the heat exchangers, as viewed in the direction of flow of the partial flow, for reliable cooling of blades of the gas turbine. A partial quantity of the cooled partial flow can be fed through the cooling-air line to the gas turbine as cooling air in order to cool the blades.
The advantages achieved with the invention reside in particular in the fact that a flexible adaptation of the gas and steam-turbine plant to different integration concepts while achieving an especially high plant efficiency is made possible by the cooling of the bleed air in a number of heat exchangers constructed as evaporators for a flow medium. In this case, the extraction of heat from the bleed air through the heat exchanger constructed as an evaporator is independent of the input of heat into the saturator circuit. The gas and steam-turbine plant can therefore be used in an especially reliable manner even in various operating states. Furthermore, the structure of the respective heat exchanger as an evaporator permits an especially simple supply of secondary consumers with process steam or with heating steam. In particular, the gasifier or a gas-processing component connected downstream of the latter is suitable as such a secondary consumer. In this case, due to the comparatively high storage capacity of the respective evaporator circuit, even fluctuating tapped quantities of process steam or heating steam by the respective secondary consumers do not lead to operational malfunctions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a gas and steam-turbine plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.